Orthogonal frequency division multiplexing (OFDM) is a popular scheme for wideband digital communications. In OFDM transmission schemes a stream of data symbols to be transmitted is divided into a plurality of sub-streams, and each of these sub-streams is modulated onto one of a plurality of mutually orthogonal sub-carriers within the available transmission bandwidth using a modulation scheme such as quadrature amplitude modulation (QAM), quadrature phase shift keying (QSK) or binary phase shift keying (BPSK). The plurality of modulated sub-carriers are linearly summed to generate the signal to be transmitted.
Whilst OFDM schemes offer benefits such as improved spectral efficiency and reduced susceptibility to inter-symbol interference, they suffer from the disadvantage of a high peak to average power ratio (PAPR). In order to minimise distortion and other negative effects associated with amplifier non-linearities, it is important that a power amplifier in an OFDM transmitter operates in its linear region as much as possible. However, as the peak power of the OFDM signal to be transmitted may be significantly higher than its average power, due to differences in the power of individual data symbols in the signal to be transmitted, the amplifier must be configured such that it will operate in its linear region for the full range of input symbol powers, i.e. at both the average symbol power and the peak symbol power. This implies configuring the amplifier with a significant back-off, i.e. the output signal power of the amplifier for an input symbol of average power is significantly lower than the maximum output power of the amplifier, so that if an input symbol of peak power is received the amplifier still operates in its linear range and is thus able to amplify the input symbol without introducing distortion in the amplified output signal.
This back-off may be as much as 10 dB, which means that for an amplifier that is capable of an output power of 1 Watt, the useful power output is only 100 milliwatts. This significantly impairs the power efficiency of the OFDM transmitter.
In practice, the choice of back-off is typically a compromise between amplifier efficiency and amplifier linearity, and typically an amplified symbol output by a power amplifier in an OFDM transmitter will be clipped due to the non-linearity of the amplifier. This clipping of the output signal gives rise to spectral regrowth and transmit errors. This is because the clipping reduces the amplitude of symbols to be transmitted, causing their actual position (in a polar symbol space) to differ from the expected position of the symbol in the polar symbol space. Spectral regrowth occurs when the actual position of a symbol in the polar symbol space due to clipping results in the energy of the symbol appearing outside of the transmit bandwidth of the signal, whilst transmit errors occur when the energy of the symbol appears in the transmit bandwidth of the signal. The error vector magnitude (EVM) of a transmitted symbol is the magnitude of the difference between its actual position (in a polar symbol space) and its expected position.
Higher order modulation schemes require very low EVM values. For example, for good operation of the 256QAM, R=5/6 modulation and coding scheme (which is known as MCS9) used in the IEEE802.11ac standard, the total EVM should be no greater than −36 dB in average white Gaussian noise (AWGN) channel conditions. This −36 dB figure is the total EVM budget, and so the transmit errors resulting from clipping should be lower, as there are other sources of EVM such as phase noise in a local oscillator of the transmitter.